1. Field of the Invention
The present invention relates to a semiconductor memory device. More particularly, it relates to a semiconductor memory device having memory cells arranged therein, each of which comprises a variable resistive element configured to store data based on variation in resistance of the variable resistive element.
2. Description of the Related Art
Conventionally known and marketed semiconductor memory devices such as DRAM, SRAM, and flash memory each use a MOSFET as a memory cell. Consequently, there is required, accompanying the miniaturization of patterns, an improvement in dimensional accuracy at a rate exceeding a rate of the miniaturization. As a result, a large burden is placed also on the lithography technology for forming these patterns which is a factor contributing to a rise in product cost.
In recent years, resistive memory is attracting attention as a candidate to succeed these kinds of semiconductor memory devices utilizing a MOSFET as a memory cell (refer, for example, to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-522045). The resistive memory devices herein include resistive RAM (ReRAM), in a narrow sense, that uses a transition metal oxide as a recording layer and stores its resistive states in a non-volatile manner, as well as Phase Change RAM (PCRAM) that uses chalcogenide or the like as a recording layer to utilize the resistance information of crystalline states (conductors) and amorphous states (insulators).
It is known that the variable resistance elements in resistive memory have two modes of operation. One is to set a high resistive state and a low resistive state by switching the polarity of the applied voltage, which is referred to as “bipolar type”. The other enables the setting of a high resistive state and a low resistive state by controlling the voltage values and the voltage application time, without switching the polarity of the applied voltage, which is referred to as “unipolar type”. The memory cell is in a stable state in the high resistive state (the reset state), for example. If the memory cell stores 2-value data, data write thereto is performed by a setting operation that changes the reset state to a low resistive state.
In a semiconductor memory device with memory cells each having such a variable resistor element arranged therein, a high voltage may be applied to a wiring, a diode, or the like, after the variable resistor element is switched from a high resistive state to a low resistive state. Such a high voltage might accelerate deterioration of diodes or the like.
In addition, variation in the resistance value of the variable resistor element may prevent voltages necessary for various parts from being applied thereto. Alternatively, it may cause a high voltage more than required to be applied thereto. This may cause malfunction or increase in power consumption.